The present invention relates generally to the field of electronics manufacturing, and more specifically to an improved apparatus and method for transporting substrates, such as circuit boards, in a screen or stencil printer that prints solder paste on a surface of the substrates.
In a typical surface mount electronic circuit board manufacturing facility, a number of independent machines, each of which performs a different operation, are used in an assembly line to manufacture circuit boards, One typical example of a circuit board assembly line 10 is shown in block format in FIG. 1. The assembly line includes a circuit board loader 12, a stencil printer 14, a pick and place machine 16 and a reflow oven 18. The machines in the assembly line are interconnected using a number of conveyor segments 20.
In the assembly line 10, the circuit board loader 12 provides a bare circuit board to the stencil printer wherein solder paste is printed onto pads of the circuit board in a predetermined pattern defined by a stencil loaded into the stencil printer. The stencil printer then passes the circuit board to the pick and place machine 16, wherein electrical components are placed onto the solder paste on the circuit board. Next, the circuit board is passed to the reflow oven, wherein the circuit board is subjected to a sufficiently high temperature to cause the solder paste to flow to secure the components to the board and provide proper electrical connections between the components and the circuit board.
One example of a stencil printer 14 is shown in FIG. 2. Stencil printers, like that shown in FIG. 2, are available from MPM Corporation, Franklin Mass. the assignee of the present invention, and include model numbers ULTRAPRINT 2000, ULTRAPRINT 2500 and ULTRAPRINT 3000. The stencil printer 14 includes a frame 22 that supports components of the stencil printer including a controller 24 with a user interface 26, a stencil 28, and a dispensing head 30 having a dispensing slot 32 from which solder paste is dispensed. When a circuit board 36 is loaded into the stencil printer and properly aligned in the printer, a z-axis work table lifts the circuit board to place the circuit board against, or in close proximity with, the stencil of the printer so that printing can occur. The dispensing head 30 is then placed over the stencil and moved across the stencil while solder paste is dispensed through the slot 32, causing the solder paste to be printed onto the circuit board. The operation of the dispensing head 30 is further described in co-pending U.S. patent application Ser. No. 08/966,057 (now U.S. Pat. No. 5,947,022) to Freeman et. al, which is assigned to the assignee of the present invention, and incorporated by reference herein. As known to those skilled in the art, some prior art stencil printers utilize one or more squeegee blades with a paste dispensing mechanism in place of the dispensing head described above.
The stencil printer 14 includes a conveyor 34 for loading/unloading and for positioning circuit boards in a print area above the z-axis work table of the screen printer. The conveyor 34 of the stencil printer is positioned to receive circuit boards 36 from and deliver circuit boards 36 to conveyor sections 20 of an assembly line, as shown in FIG. 1, to move circuit boards 36 between the machines in the assembly line. Typical inline circuit board manufacturing machines, such as those shown in FIG. 1, communicate with each other using standard protocols defined by the Surface Mount Equipment Manufacturer""s Association (SMEMA) to transfer substrates such as circuit boards between the machines. For example, using SMEMA protocols, the stencil printer 14 signals to the pick and place machine that it has a circuit board 36 ready to transfer to the pick and place machine, and when the pick and place machine signals to the stencil printer that it is ready to receive the circuit board 36, then the circuit board 36 is transferred over conveyor section 20.
To reduce the time and conveyor travel distance required to load and unload circuit boards, in some prior art stencil printers, conveyor 34 in the stencil printer is comprised of three independently controllable conveyor segments 34a, 34b and 34c. The three segments define three circuit board positions identified as a pre-print position 38, a print position 40 and a post-print position 42. Circuit boards are initially loaded into the pre-print position 38, move to the print position 40 within the print area 44 of the screen printer, and after printing, move to the post-print position 42 to wait to be unloaded from the stencil printer. The use of independently controllable conveyor segments allows circuit boards to be loaded and unloaded in a more flexible manner to meet demands of the other machines of an assembly line without waiting for a print cycle to be completed, thereby lowering the idle time of the machines, and allowing the assembly line to operate in an asynchronous manner.
Although the use of three conveyor segments for the conveyor in a stencil printer has advantages over the use of a single conveyor segment, this approach is relatively expensive, is complex and typically reqires a significant amount of space for the mechanical and electrical components used to implement the three conveyor segment approach.
Embodiments of the present invention provide a method and apparatus for transporting circuit boards in a screen printer having the advantages of a three conveyor segment conveyor system, without the above-described disadvantages.
In a first embodiment of the present invention, a transportation system is provided for a printer that prints material on a substrate. The transportation system loads substrates into the printer, unloads substrates from the printer and transports substrates between positions within the printer. The transportation system includes a conveyor disposed within the printer that receives a substrate to be printed on, and that moves the substrate to predetermined substrate positions in the printer, wherein at least one of the predetermined substrate positions is a print position at which the material is printed onto the substrate, and means for independently controlling movement of a plurality of substrates on the conveyor.
In one version of the first embodiment, the means for independently controlling movement includes at least one device that lifts at least one substrate off of the conveyor such that the substrate does not move in response to movement of the conveyor.
In another version of the first embodiment, the means for independently controlling includes a least one pin having an activated position in which at least one substrate is prevented from moving beyond the pin on the conveyor.
Another embodiment of the present invention is directed to a method of printing material on substrates using a printer having a transportation system that transports the substrates to positions within the printer. The method includes steps of transferring a first substrate to a print position in the printer using the transportation system, raising the first substrate off of the transportation system to print the material on the substrate, transferring a second substrate to a pre-print position in the printer while the first substrate is raised off of the transportation system and raising the second substrate off of the transportation system, transferring the first substrate to a post-print position and raising the first substrate off of the transportation system, lowering the second substrate onto the transportation system, transferring the second substrate to the print position and raising the second substrate off of the transportation system to print material on the second substrate, and while the first and second substrates are raised above the transportation system, transferring a third substrate to the pre-print position and raising the third substrate off of the transportation system.
The method may further include steps of lowering the first substrate back onto the transportation system, and transferring the first substrate from the post-print position.